Accurate crystallographic methods developed over the last decade are to be applied in the mapping of the three dimensional electron density distribution of iron porphyrins. The compounds selected are models for the prosthetic group of oxygen-transport and electron-transfer hemoproteins, and represent different oxidation states spin states and coordination numbers. In addition to increasing our fundamental knowledge of the active site of the hemoproteins the experimental electron density distribution will answer a number of specific questions related to the extent to which changes in electronic structure of the heme contribute to the cooperativity mechanism for loading and unloading of oxygen. Differences in radial dependence of the transition metal electron distribution and the nature of the bonding between metal and ligands will be analyzed. Knowledge of the electronic structure of the porphyrin ligand will provide basic information which can be applied to the larger number of other biological systems in which porphyrin complexes participate. The experimental electron density distribution will serve in the interpretation of the large body of physico-chemical data collected in recent years. The research will be performed mainly on automated low temperature diffraction equipment in the principal investigators laboratory. Use will be made of an extensive library of locally developed programs for charge density analysis by diffraction methods. In a later stage part of the measurements will make use of the SUNY beamline at the National Synchrotron Light Source at Brookhaven National Laboratory which is to become operational during 1983.